Noise generation due to pressure pulsations is a natural phenomenon in undamped positive displacement compressors used in HVAC systems as well as other applications such as pipeline applications, as discrete volumes of gaseous fluid enter a chamber at a low, suction pressure, are compressed to a high pressure and are then discharged from the chamber at a high discharge pressure. The periodic suction and discharge of the gaseous fluids is a pulsation event that produces a vibration. At certain frequencies, about 20 to 20,000 Hz, these vibrations fall within the audible range for humans and are perceived as noise. Of course, vibrations are readily propagated along the metal surfaces that comprise the pipes, conduits and other equipment through which the gaseous fluid is circulated.
When the periodicity of the vibrations changes, as a result of change in, for example, the speed of operation of the variable speed compressor, the frequency of vibration also changes. Some noise at certain frequencies resulting from the operation of positive displacement compressors may be less annoying than other frequencies. While it is desirable to completely attenuate the noise generated by operation of a positive displacement compressor, sometimes this is not possible. Mufflers are added to either or both the suction side (low pressure side) of the compressor or the discharge side (high pressure side) of the compressor. While mufflers ideally attenuate sound to eliminate noise, in practice mufflers are designed to tune the sound that is propagated so that sound in certain undesirable frequencies, typically the most annoying frequencies, is attenuated. Thus, these mufflers or resonators are designed to target a fixed frequency and cannot be adjusted readily. To change the target frequency, the muffler physically must be removed from the system and physically modified or replaced with a muffler or resonator designed for a different fixed frequency. Physical modifications to a resonator can require removal of the resonator from site and returning it to the manufacturer. The periodicity of vibrations produced by a positive placement compressor is variable and may change with load, which can vary not only from season to season, but also from day to day, depending upon the application. The frequency range attenuated is generally limited. However, mufflers are designed to attenuate predetermined frequencies. Thus, mufflers can become ineffective as the periodicity of vibrations changes with the speed of operation of the compressor. What is desired is an attenuating apparatus that can dampen noise across a range of frequencies and that readily can be adjusted to attenuate noise at preselected frequencies within the range of frequencies, as conditions warrant.